U.S. Pat. No. 3,712,736 to Boncoeur et al is concerned with testing welded work pieces by optical spectroscopy wherein a wire or film of a tracer element diffuses within a work piece when it is in a liquid state, and the presence of the tracer in plasma above the weld zone is continuously detected by spectroscopy.
U.S. Pat. No. 4,120,196 to Hamilton et al discloses a sensing device which monitors the distance to a selected wearing cutting surface of a tool and the distance to a selected non-wearing surface of the tool. U.S. Pat. No. 4,514,797 to Begin is likewise concerned with a worn tool detector system in which input signals from a sensor in the form of an analogue waveform exhibiting periodic impulses exceeding the average amplitude thereof are analyzed as the tool wears.
U.S. Pat. No. 4,620,185 to Plahmer describes a machine condition diagnostic system wherein machine components are labelled with wear indicator materials. The lubricant is examined and the presence of a particular indicator in abnormal amounts directs attention to a precise location in need of repair or replacement.
U.S. Pat. No. 4,774,150 to Amano et al provides a thermal barrier coating which allows nondestructive testing to determine whether the coating has a thickness sufficient for thermal barrier effect.